Knurled closure

ABSTRACT

A method of forming an annular knurled band on two surfaces of a container closure by providing a generally cylindrical closure shell having a top panel and a depending annular skirt. Forming a plurality of outwardly directed knurled portions within a plurality of first circumferential arcs by outwardly reforming the arcs to establish in each knurled section a lower portion extending radially outwardly from the annular skirt and an upper portion formed as a continuous extension of the lower portion, extending upwardly and outwardly from the top panel. Forming a plurality of inwardly directed knurl portions in second circumferential arcs in alternating position with the outwardly directed knurl portions by restraining the second arcs against outward movement. The closure having both the outwardly directed and the inwardly directed knurled portions each of substantially uniform transverse width throughout their longitudinal extent. The knurling originates within the upper half of the skirt and terminates at its uppermost level in the upper panel.

United States Patent [1 1 Willis [45]- Feb. 20, 1973 [54] KNURLEDCLOSURE Primary Examiner-Joseph R. Leclair 75 Inventor: Wilburn CoyWillis, Richmond, 1nd. Marcus Attorney-Arnold B. Sllverman [73]Assignee: Aluminum Company of America,

Pittsburgh, Pa. 57 ABSTRACT Filed: March 29, 1971 A method of forming anannular knurled band on two [62] Division of Ser. No. 807,189, March 14,1969, Pat.

[52] U.S. Cl. ..215/43 R, 215/46 R [51] Int. Cl. .L ..B65d 41/04 [58]Field of Search ..2l5/42, 43, 44, 46 R, 46 A; 113/121 A [56] ReferencesCited UNITED STATES PATENTS 2,980,274 4/1961 Gould ..2l5/43 R 3,123,2413/1964 Nofer et al. ..215/39 1,160,596 11/1915 Hammer ..215/44 3,158,27811/1964 Knapp et al. ..215/42 1,495,722 5/1924 Anderson ..2l5/43 R3,433,380 3/1969 Kawchitch ..215/43 R 1,942,810 1/1934 Gibbs ..215/43 Rsurfaces of a container closure by providing a generally cylindricalclosure shell having a top panel and a depending annular skirt. Forminga plurality of outwardly directed knurled portions within a plurality offirst circumferential arcs by outwardly reforming the arcs to establishin each knurled section a lower portion extending radially outwardlyfrom the annular skirt and an upper portion formed as a continuousextension of the lower portion, extending upwardly and outwardly fromthe top panel. Forming a plurality of inwardly directed knurl portionsin second circumferential arcs in alternating position with theoutwardly directed knurl portions by restraining the second arcs againstoutward movement. The closure having both the outwardly directed and theinwardly directed knurled portions each of substantially uniformtransverse width throughout their longitudinal extent. The knurlingoriginates within the upper half of the skirt and terminates at itsuppermost level in the upper panel.

5 Claims, 7 Drawing Figures PATENTED'FEBZO 191s 3,717 27 5 sum 1 or 2-uvvewron. W/LBURN cor WILL/S wam Attorney KNURLED CLOSURE Thisapplication is a division of parent application Ser. No. 807,189, filedMar. 14, 1969, now U.S. Pat. No. 3,623,442.

BACKGROUND OF THE INVENTION 1 Field of the Invention This inventionrelates to knurled closures and. more specifically to torque removableclosure knurling having portions of outstruck enlarged diameter withrespect to the closure shell diameter and a continuous annular knurledband formed partially within the closure skirt and partially within theclosure top panel.

2. Prior Art It is known in the closure art to form various types ofstructural irregularities which facilitate manual engagement forapplying a closure removal torque. Such irregularities frequently takethe form of knurling which is provided in the closure skirt atcircumferentially spaced positions.

Knurling may be provided in various ways. In plastic closures, forexample, it may be integrally molded. In metal closures it isconventionally provided by applying a radially inwardly directed forceto reform the closure skirt inwardly at spaced intervals about thecircumference of the closure. The relief provided in this fashionproduces knurled portions having a maximum diameter equal to or slightlyless than the diameter of the original closure shell. The diameter oftherecessed portions is substantially less. As such knurling is limitedin maximum diameter by the closure diameter, only use of a largerdiameter closure shell could provide an enlarged knurl.

It has also been known in some instances to employ outwardly directeddeformation of a closure skirt to provide an enlarged knurl diameter.This provides some advantages over the objectionable features of theinwardly directed knurl mentioned above. 7

In order to improve the effectiveness of the knurling, it is frequentlyextended over the entire skirt. As the knurling is generally employed inclosures having screw threads or lugs, the objective of maximum knurllength conflicts with the objective of obtaining efiective threads orlugs. The knurls are, however, frequently extended downwardly over thethreads.

There is lacking an effective method for obtaining a maximum efficiencysimple knurl structure in a torque operated closure which avoids theabove mentioned problems.

SUMMARY OF THE INVENTION This invention provides a method of forming aclosure provided with an annular knurled band having the outwardlydirected knurled portion extending radially outwardly beyond thediameter of the closure shell or workpiece over an upper portion of theclosure skirt and a portion of the closure top. The knurled bandconsists of outwardly directed knurled portions disposed in alternatecircumferential position around the closure skirt with respect toinwardly directed knurled portions. The outwardly directed portions arepreferably of generally V-shaped transverse cross sectionalconfiguration, while the inwardly directed portions are of substantiallystraight or flat transverse cross sectional configuration. While, as apractical matter, there will be a slight curve to the inwardly directedportion, this would be considered to be substantially flat herein. Eachoutwardly directed portion is formed within a first circumferential arc.The outwardly directed portions are preferably formed within firstcircumferential arcs totaling about 10 to 20 percent of the totalcircumference of that portion of the closure shell within which theannular knurled band is formed. The knurled portions of this inventionmay originate in the upper half of the closure skirt and extendcontinuously upwardly and curvingly radially inwardly through the top ofthe skirt and into the top panel.

The method of this invention provides for formation of alternateindividual outwardly and inwardly directed knurl portions. The firstcircumferential arc is formed radially outwardly while the adjacentsecond circumferential arcs are restrained against radial outwardmovement. In this fashion, first circumferential arcs are moved radiallyoutwardly along the skirt portion and upwardly and outwardly along theclosure top panel. For a given outwardly directed knurl portion this isaccomplished substantially simultaneously. The same restraining actionalong the length produces substantially simultaneous formation of anindividual inwardly directed knurl portion over its entire length.

The total number of outwardly and inwardly directed knurl portionscombine to form the annular knurled band which may be formed eithersimultaneously or progressively.

The total circumferential area covered by the first arcs is less thanthat covered by the second arcs. Preferably the first arcs cover a totalof about 10 to 20 percent of the total circumference of that portion ofthe closure shell which will be occupied by the annular knurled band.

The transverse width of the outwardly directed knurl portions ispreferably uniform throughout their length as is the case with respectto the transverse width of the inwardly directed knurl portions. Theheight of the outwardly directed knurl elements preferred exceeds theinwardly directed elements by about 0.040 to 0.060 inch.

The closure top panel has an inner portion, which is preferablysubstantially flat, and an annular outer portion which curves downwardlyand radially outwardly to meet the closure skirt. The lower origin ofthe knurled band originates on the closure skirt at a position withinthe upper half and preferably the upper quarter of the closure skirt.The band extends initially substantially vertically upwardly and thencontinues over at least a major portion of the annular outer portion ofthe top panel. The origin of the knurled band is above any thread or lugelements,'and terminates in the top panel at the uppermost portion ofthe band.

The outwardly directed knurl sections are of substantially V-shapedtransverse cross sectional configuration and the inwardly directed knurlportions are preferably of substantially flat transverse cross sectionalconfiguration.

The annular knurled band also serves to rigidify and protect the impactsusceptible upper corner of the closure.

It is an object of this invention to provide a knurled closure having animproved enlarged knurled structure which may be more readily manuallyengaged for closure application or removal, and a method ofmanufacturing the same.

It is another object of this invention to provide a knurled torqueremovable closure which may be more effectively engaged by sealingmachinery for securing a threaded or lug type closure to a container.

It is another object of this invention to provide a closure havingcontinuous knurl portion of uniform transverse width throughout theirlength.

It is yet another object of this invention to provide a method ofmanufacturing a unitary metal closure providing an outwardly formedknurled band which provides protective rigidity to the closure in theregion where the top panel connects with the depending skirt.

Other objects and advantages of this invention will be understood fromthe following description of the invention on reference to the drawingsappended hereto.

BRIEF DESCRIPTION OF THE DRAWINGS ing an embodiment of this invention.

FIG. 2 is an elevational view, partially in section, of a closure ofthis invention.

FIG. 3 is a plan view partially in section of a closure of thisinvention.

FIG. 4 is a detailed fragmentary sectional view of a portion of theclosure shell out of which the closure will be formed.

FIG. 5 shows an intermediate stage of the method of the invention withthe outwardly directed knurls being partially formed.

FIG. 6 shows another stage of the method of this invention with oneknurl completely formed.

FIG. 7 is a fragmentary sectional elevation showing the tooling inengagement with both the closure skirt and the top panel.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now more specificallyto the drawings, FIG. 1 shows a threaded closure 2 which has a skirt 4depending from a panel 6. An annular knurled band 8, in the formillustrated, has the knurled elements originating in the upper portionof the closure skirt 4 and extending continuously upwardly through thetop of the skirt and into the closure panel 6. As is shown in FIGS. 2and 3, the knurled band 8 has outwardly directed knurled portions 14which are uniformly positioned around the circumference of the closurein alternate positions with respect to the inwardly directed portions16.

As is shown in FIGS. 2 and 3, the outwardly directed knurled portions 14establish a diameter D, which is greater than diameter D, which was theoriginal diameter of the closure shell skirt. The inwardly directedknurled portions 16 have a diameter D substantially equal to theoriginal shell skirt diameter. The knurled structure, therefore,provides a minimum diameter D approximating that of the original shelland an enlarged diameter D, exceeding that of the original shell. FIG.7, which is a fragmentary elevation of the closure positioned in theforming dies, shows an inwardly directed knurl portion in the solidlines and an outwardly directed knurl portion in the dotted lines. Thetop panel 6 has an annular outer portion (defined by the letter T) whichsurrounds the substantially flat inner top panel portion. The annularouter portion T has a curved downwardly and radially outwardlyconfiguration and terminates at its lower end by merging into theclosure skirt 4.

As is shown in FIGS. 1 through 3 and 7, the annular band defined byknurl portions 14, 16, therefore, has a lower origin within the upperone half of the closure skirt and extends continuously upwardly throughat least a major portion of the annular outer portion of top panel 6.The terminus of the band is disposed at the uppermost portion thereof,within the top panel 6. From the origin the band extends initiallyvertically upwardly within the skirt 4 and then curves upwardly andradially inwardly within the annular outer portion T of the top panel 6.While the band may continue into the inner top panel portion, it ispreferred that the terminus be positioned in relatively close proximityto the radially innermost end of the annular outer portion T.

In this fashion, the two surface coverage by the knurled portions 14,16, which cover a relatively small portion of the closure, provides anoutwardly enlarged continuous closure knurl of definite extent whichfacilitates machine engagement for applying the closure to a containeras well as manual engagement for removal thereof. In addition, itprovides an annular reinforced region at the area of joining of the toppanel 6 and skirt 4 to provide increased resistance to the closure 2.

The term torque removable closure as used herein shall refer to aclosurehaving continuous or discontinuous threads, lugs or othercontainer finish engaging protrusions which require the application oftorque to apply or remove the closure from a container. It is notimportant whether the container finish engaging protrusions are formedbefore knurling, simultaneously therewith or thereafter.

In FIG. 4 a section of a generally cylindrical closure shell skirt isshown; the dotted lines reflecting the outwardly curved knurl elements14 to be formed and the intermediate inwardly directed knurl elements 16shown in solid lines. A series of first circumferential arcs A are ofsubstantially the same size and disposed in circumferential alternatingposition with respect to a series of second circumferential arcs A,which are also all of substantially the same size. These arcs A A,,while for convenience of .illustration are shown in the skirt, will beunderstood to extend upwardly therefrom into the top panel 6 to theterminus of the annular knurled band. Circumferential arc A provides themetal to be used in forming element 14 of height h. Arc A, which islonger than arc A provides the metal for forming element 16.

By means of the method of this invention, the outwardly directed knurlelements 14 are formed by outwardly reforming arc A, to a height h. Thisgenerally involves stretching or thinning of the metal in are A,, Itwill be appreciated that when each knurl portion 14 is created from anare A, the entire portion 14 may be formed substantially simultaneouslyby reforming the entire arc A from the lower origin within the skirt 4to the upper terminus within the top panel 6. The lower sector of theknurl portion 14 is within the skirt 4 and is disposed substantiallyvertically while the upstanding upper portion which is formed as acontinuous extension of the lower portion extends upwardly and radiallyinwardly into the top panel while being raised with respect thereto. Theannular knurled 'band preferably has its origin within the upper quarterof the closure skirt at a position above the thread elements on theclosure.

The inwardly directed knurl portions 16 are formed by substantiallycompletely resisting movement of arc A during formation of portions 14.In this fashion portions 16 have a diameter D, substantially equal tothe original shell diameter and portions 14 have a diameter D,equivalent to double the height h of portions 14.

By means of the method of this invention, the larger element 16 isformed without substantial movement of the metal in arc A Smaller arc Ais merely formed radially outwardly to establish element 14. Thus,instead of overcoming the substantial resistance required to reformlarge arc A, inwardly, to establish knurled band 8, smaller arc A ismerely reformed radially outwardly. The difference in length of arcs A Ais significant as it is easier to reform the smaller arc A radiallyoutwardly to form smaller element 14, than to reform larger arc Aradially inwardly to form element 16. By proper selection of the lengthsof A A, the total number of elements 14, 16 can be determined, as wellas the height h of the element, for any closure of given diameter. Theheight h of element 14, of course, determines the increase in diameter Dover D This may be slightly varied by permitting some movement of Adurin g the forming stages, but this is not essential.

One significant feature of this invention is the permitted variations inthe diameter D of the outwardly directed knurled elements 14 withrespect to height h and circumferential frequency and width of elements14, 16. While the beneficial results of this invention can be obtainedwith are A larger than are A the preferred arrangement provides a totalcircumferential extent of arcs A, equal to about to 20 percent of thetotal circumference of the closure shell. The total circumferentialextent of all of arcs A, then equals about 80 to 90 percent of the shellcircumference. This results in outwardly directed knurled sections 14being of a readily formable size, and provides a desired section heighth, coupled with sufficient circumferential gap between adjacent sections14 to facilitate gripping. The preferred exterior height h of hollowsections 14 with respect to sections 16 is about 0.020 to 0.030 inch,which results in diameter D, exceeding diameter D, by a maximum of about0.040 to 0.060 inch. The height h may vary along the length of the knurlportion 14, but will reach a maximum intermediate opposed ends thereof.

The formation of annular knurled band 8 may be effected in the mannershown in FIGS. 5 through 7. Inner die member 20 has bosses 22 which areadapted to cooperate with recesses 26 of outer die 24. In FIG. 5,forming has been initiated, while in FIG. 6 the elements 14, 16 havebeen formed. In FIG. 5, center boss 22 of inner die 20 has initiateddeformation of the one outwardly directed knurl portion. It is notedthat portions of die 24 abut the adjacent arcs A, in surface to surfacecontact which resists outward movement thereof. In

FIG. 6, the formation of the center knurl portion 14 is origin to toppanel terminus. The terminus may be within the annular outer portion oftop panel 6 or within the inner top panel portion in close proximity toportion T. Little if any added benefit is obtained by extending theterminus to a position toward the center of top panel 6. Outer die 24 isprovided with an extension 28 which incorporates a curved transition anda top panel contacting surface. Inner die 20 is provided with a panelsupporting surface 30. Extension 28 and surface 30 have complementaryportions which are continuations of bosses 22 'and recesses 26.

It will be appreciated that as a result of this method of formingknurling by moving metal outwardly with respect to the original closureshell, the top extension 28 serves as an added restraint whichcontributes to effective formation of continuous knurling on the skirtand top panel. The outer die 24 therefore provides restraining surfacesengaging the skirt to resist radial expansion where elements 16 are tobe formed and also to resist, through extension 28, upward deformationof the upper sector of elements 16.

In the form illustrated, while the individual portion 14, such as thecenter one in FIGS. 5 and 6, is substantially simultaneously formed, theadjacent arcs A are in a different stage of formation. The formation ofthe annular knurled band is therefore being effected progressivelyaround the circumference of the closure. This process may convenientlyalso be effected simultaneously by means of continuous dies orprogressively as by roller type dies. In general, it should be notedthat increased speed of production would be obtained by simultaneousformation. Progressive formation of roller type dies may be accomplishedat an increased speed by employing dies of sufficient size to haveseveral outwardly directed elements 14 in a forming stagesimultaneously. Also, the use of progressive roller type dies providesthe distinct advantage of being able to combine the knurling operationwith a progressive roller type threaded or lug formation, therebyeliminating the need for a separate knurling station.

With respect to configuration, the outwardly directed knurled sections14 may have a suitabletransverse cross sectional configuration which isgenerally V-shaped. The inwardly directed knurled sections 16 preferablyhave a straight or flat transverse cross sectional configuration. (As apractical matter, the sections 16 will generally be an arc A which has aradius approximating that of the closure skirt, but this, for purposesof the present description, will be considered as being generally flator straight.)

One problem frequently encountered in the hoppering and handling ofclosures prior to affixation is undesired nesting or interlockingcreated by the top of one closure entering into the opening in thebottom of another. This frequently impedes proper feeding of closuresand requires a brief cessation of production. The outwardly projectedknurled band of this invention provides an enlarged closure top whichresists such nesting and interlock.

The closure is preferably made from a unitary deformable substantiallyrigid shell which may conveniently be of metal, such as aluminum or tinplate.

While it forms no part of this invention and has for purposes of clarityof illustration not been shown, it is of course contemplated that theclosure may be provided with a suitable sealing gasket such as a disc,ring, inner shell or other convenient form.

It will be appreciated that the annular knurled band provides arigidified corner structure which will increase the impact resistance ofthe closure 'in this damage susceptible area.

It will be appreciated that as a result of diameter D being greater thandiameter D the torque arms are larger, thereby providing a finiteincrease in mechanical advantage for one turning the closure end toremove it or to secure it on the container. The benefit is obtainedsimply and without the need to employ additional material in the closureshell.

It will, therefore, be appreciated that the method and closure of thisinvention provide an economical means for effecting an improved knurledclosure structure. Without requiring additional metal in the closureshell or workpiece, the closure skirt may be provided with an upstandingannular knurl band of increased diameter. This knurl may have apredetermined height h made from predetermined arc widths (A A Theknurling may originate in the upper half, preferably the upper quarter,of the skirt portion and extend continuously upwardly through theclosure panel. The added height of the knurls and presence in both skirtand top panel result in a knurled band having a greater difference inheight between the sections 14, 16 and an enlarged diameter D Thisfacilitates more effective engagement by hand or machine to apply torqueto the closure in effecting application or removal. Also, the increaseddiameter provides an increased mechanical efficiency due to the addedtorque arm length. All of this is accomplished simply, while providing adistinctive closure of improved appearance and functioning.

While for purposes of clarity of expression and full disclosure, certainspecific forms of the invention have been expressly referred to, itwould be appreciated that numerous variations may be made withoutdeparting from the scope of the invention.

1 claim:

1. A torque removable container closure, comprising a top panel havingan inner panel portion and an annular curved outer panel portion whichis disposed radially outwardly of said inner panel portion and extendsdownwardly and radially outwardly,.

an annular skirt portion depending from said annular curved outer panelportion,

an annular knurled band having circumferentially altemating outstruckoutwardly directed hollow knurl sectors and inwardly directed knurlsectors,

said outstruck outwardly directed hollow knurl sectors having an averagewall thickness less than the average wall thickness of said inwardlydirected knurl sectors,

said knurled band having a lower origin within the upper one half ofsaid annular skirt and extending continuously upwardly and terminatingadjacent the outermost edge of said inner panel portion of said toppanel,

said knurled band having a lower sector extending substantiallyvertically upwardly from said origin and an upper sector extendingcurvingly upwardly and radially inwardly from said lower sector to aterminus at the up ermost ortion of said band said outwardly direc edknur sectors are of substantially uniform transverse width throughouttheir length,

said inwardly directed knurl sectors are of substantially uniformtransverse width throughout their length, and

said outwardly directed knurl sectors are of generally V-shapedtransverse cross sectional configuration.

2. The container closure of claim 1 wherein said inner panel portion ofsaid top panel is substantially flat,

said annular knurled band having said origin disposed within the upperquarter of said annular skirt, and

said inwardly directed knurl sectors being of substantially flattransverse cross sectional configuration.

3. The container closure of claim 2 including said closure is a unitarystructure made of metal,

inwardly directed thread means within said closure skirt, and

said annular knurled band disposed above said thread means.

4. The container closure of claim 3 wherein said metal is aluminum, and

the diameter at said annular skirt of said outwardly directed knurlportions exceeds the diameter of said annular skirt at said inwardlydirected knurl portions by 0.040 to 0.060 inch.

5. The container closure of claim 1 wherein said knurled band terminatesat said outermost edge of said inner panel portion.

UNITED STATES PATENT OFFIGE 1 CERTIFICATE OF CORRECTWN Patent No.3,7171275 Dated February 20, 19-73 Inventor(s) 11 1b m C 'HilliS It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Col. 2, line 41 After "about" change "0.040 to 0O60" to --0.020 to0.030--.

Col. 3, line 6 After "knurl" change "portion" to v --portions--. Col. 4,line 42 After "arcs" change "A" to -A Col. 5, line 9' I After "to"insert -D p 1us-.

Col. 5, line 26' After "diameter" change "D over D to "D2 over D1--.

Signed and sealed this 3rd day of July 1973.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. Rene Tegtmeyer Y Attestlng Officer ActingCommissioner of Patents FORM PO-105O (10-69) I USCOMWDC 6o376 p69 I flU,S. GOVERNMENT PRINTING OFFICE: I969 O-366-3 34

1. A torque removable container closure, comprising a top panel having an inner panel portion and an annular curved outer panel portion which is disposed radially outwardly of said inner panel portion and extends downwardly and radially outwardly, an annular skirt portion depending from said annular curved outer panel portion, an annular knurled band having circumferentially alternating outstruck outwardly directed hollow knurl sectors and inwardly directed knurl sectors, said outstruck outwardly directed hollow knurl sectors having an average wall thickness less than the average wall thickness of said inwardly directed knurl sectors, said knurled band having a lower origin within the upper one half of said annular skirt and extending continuously upwardly and terminating adjacent the outermost edge of said inner panel portion of said top panel, said knurled band having a lower sector extending substantially vertically upwardly from said origin and an upper sector extending curvingly upwardly and radially inwardly from said lower sector to a terminus at the uppermost portion of said band, said outwardly directed knurl sectors are of substantially uniform transverse width throughout their length, said inwardly directed knurl sectors are of substantially uniform transverse width throughout their length, and said outwardly directed knurl sectors are of generally V-shaped transverse cross sectional configuration.
 1. A torque removable container closure, comprising a top panel having an inner panel portion and an annular curved outer panel portion which is disposed radially outwardly of said inner panel portion and extends downwardly and radially outwardly, an annular skirt portion depending from said annular curved outer panel portion, an annular knurled band having circumferentially alternating outstruck outwardly directed hollow knurl sectors and inwardly directed knurl sectors, said outstruck outwardly directed hollow knurl sectors having an average wall thickness less than the average wall thickness of said inwardly directed knurl sectors, said knurled band having a lower origin within the upper one half of said annular skirt and extending continuously upwardly and terminating adjacent the outermost edge of said inner panel portion of said top panel, said knurled band having a lower sector extending substantially vertically upwardly from said origin and an upper sector extending curvingly upwardly and radially inwardly from said lower sector to a terminus at the uppermost portion of said band, said outwardly directed knurl sectors are of substantially uniform transverse width throughout their length, said inwardly directed knurl sectors are of substantially uniform transverse width throughout their length, and said outwardly directed knurl sectors are of generally V-shaped transverse cross sectional configuration.
 2. The container closure of claim 1 wherein said inner panel portion of said top panel is substantially flat, said annular knurled band having said origin disposed within the upper quarter of said annular skirt, and said inwardly directed knurl sectors being of substantially flat transverse cross sectional configuration.
 3. The container closure of claim 2 including said closure is a unitary structure made of metal, inwardly directed thread means within said closure skirt, and said annular knurled band disposed above said thread means.
 4. The container closure of claim 3 wherein said metal is aluminum, and the diameter at said annular skirt of said outwardly directed knurl portions exceeds the diameter of said annular skirt at said inwardly directed knurl portions by 0.040 to 0.060 inch. 